Despite the dearth of mechanistic insights into precisely how cell therapy works, preclinical studies commonly report a reduction in fibrosis. Thus, to understand how cell therapy limits remodeling, we must first appreciate how reparative cells interact with the stromal compartment. In the normal heart, fibroblasts are essential in maintaining the extracellular matrix. Following infarction, fibroblasts assume an active role in acute wound healing. Although this phenotypic activation is necessary for the acute post-ischemic injury response, fibroblasts become chronically activated, which contributes to post-ischemic pathology. Project 3 will elucidate whether and how cardiac mesenchymal cells (CMCs) interact with the recipient heart and will identify CMC- mediated changes in fibroblast activation. Because the post-MI heart is characterized by a shift in the balance of hyaluronan (HA) metabolism in which more HA is produced than is degraded, HA accumulation may contribute to persistent fibroblast activation and support unresolved inflammation. Furthermore, we reason that cell therapy effects myocardial repair by restoring balance to dysregulated HA metabolism, which is largely propagated by activated fibroblasts. We will perform proof-of-concept studies to show whether and how HA metabolizing enzymes in reparative cells may regulate their competence in in vivo models of post-ischemic myocardial repair. We will identify specific receptor-ligand interactions that confer reparative competence to therapeutic cells. Although the aforementioned goals are worthy of pursuit on their own, we argue they should be examined further through a translational lens because preclinical studies of reparative cells use healthy animals as cell donors; however, clinical trials of autologous cells use heart failure (HF) patients as both donors and recipients. This preclinical/clinical dichotomy creates a sizeable translational barrier. Because significant changes occur in the stromal compartment following infarction, reparative cells derived therefrom likely differ from nave reparative cells. Indeed, our preliminary data indicate that heart failure-derived CMCs lack reparative competence, which may stem from their inability to properly metabolize post-MI stromal components, such as HA. We will identify and correct defects in incompetent, heart failure-derived CMCs. Thus, our central hypothesis holds that reparative cells attenuate ventricular remodeling through recognition of and response to specific stromal components, which are lost in CMCs derived from failing hearts. We will test this hypothesis through these synergistic aims: 1) Elucidate the impact of CMCs on fibroblast activation; 2) Determine how CMCs interact with the recipient heart to limit maladaptive remodeling; 3) Identify and rescue defective mechanisms in heart failure-derived reparative cells. Thus, we will show, for the first time, how CMCs shape the post-MI stroma to limit fibroblast activation. We will also identify reasons for reparative incompetence in heart failure-derived CMCs and restore their competence by rescuing their capacity to metabolize HA. Collective insights from Project 3 will fundamentally change future cell therapy studies.